Abstract
An earlier study of Discocotyle sagittata Leuckart had suggested that the parasite may locate its host by responding to olfactory stimuli emanating from the skin of salmonids but this has now been shown to be untrue. The process of infection involved two stages: first the larvae (oncomiracidia) swim up to the surface waters in response to light and during this phase, they are uninfective; secondly the larvae lose their attraction to light, and if taken in with the respiratory current of a trout at this stage, they stand a good chance of successfully attaching to the gills. The life cycle of the parasite is shown to be synchronized with that of its host.